The ongoing search for increased aircraft engine performance has prompted materials science engineers to investigate intermetallic compounds as potential replacement materials for nickel and cobalt base superalloys currently in widespread use for gas turbine engine hardware. Of particular interest over the past decade have been gamma or near-gamma titanium aluminides as a result of their low density and relatively high modulus and strength at elevated temperatures.
Modifications have been made to the titanium aluminide composition in attempts to improve the physical properties and processability of the material. For example, the ratio of titanium to aluminum has been adjusted and various alloying elements have been introduced in attempts to improve ductility, strength, and/or toughness. Moreover, various processing techniques, including thermomechanical treatments and heat treatments, have been developed to this same end.
An early effort to this end is described in Jaffee U.S. Pat. No. 2,880,087 which discloses titanium aluminide alloys having 8-34 weight % Al and additions of 0.5 to 5 weight % of beta stabilizing alloying elements such as Mo, V, Nb, Ta, Mn, Cr, Fe, W, Co, Ni, Cu, Si, and Be. Also see Jaffee Canadian Patent 220,571.
More recent efforts to this end are described in U.S. Pat. No. 3,203,794 providing optimized aluminum contents, U.S. Pat. No. 4,661,316 providing a Ti60-70Al30-36Mn0.1-5.0 alloy (weight %) optionally including one or more of Zr0.6-2.8Nb0.6-4.0V1.6-1.9W0.5-1.2Mo0.5-1.2 and C0.02-0.12, U.S. Pat. No. 4,836,983 providing a Ti54-57Al39-41Si4-5 (atomic %) alloy, U.S. Pat. No. 4,842,817 providing a Ti48-47Al46-49Ta3-5 (atomic %) alloy, U.S. Pat. No. 4,842,819 providing a Ti54-48Al45-49Cr1-3 (atomic %) alloy, U.S. Pat. No. 4,842,820 providing a boron-modified TiAl alloy, U.S. Pat. No. 4,857,268 providing a Ti52-46Al46-50V2-4 (atomic %) alloy, U.S. Pat. No. 4,879,092 providing a Ti50-46Al46-50 Cr1-3Nb1-5 (atomic %) alloy, U.S. Pat. No. 4,902,474 providing a Ti52-47Al42-46Ga3-7 (atomic %) alloy, and U.S. Pat. No. 4,916,028 providing a Ti51-43Al46-50Cr1-3Nb1-5Co0.05-0.2 (atomic %) alloy.
U.S. Pat. No. 4,294,615 describes a titanium aluminide alloy having a composition narrowly selected within the broader prior titanium aluminide compositions to provide a combination of high temperature creep strength together with moderate room temperature ductility. The patent investigated numerous titanium aluminide compositions set forth in Table 2 thereof and describes an optimized alloy composition wherein the aluminum content is limited to 34-36 weight % and wherein vanadium and carbon can be added in amounts of 0.1 to 4 weight %. and 0.1 weight %, respectively, the balance being titanium. The '615 patent identifies V as an alloying element for improving low temperature ductility and Sb, Bi, and C as alloying elements for improving creep rupture resistance. If improved creep rupture life is desired, the alloy is forged and annealed at 1100.degree. to 1200.degree. C. followed by aging at 815.degree. to 950.degree. C.
U.S. Pat. No. 5,207,982 describes a titanium aluminide alloy including one of B, Ge or Si as an alloying element and high levels of one or more of Hf, Mo, Ta, and W as additional alloying elements to provide high temperature oxidation/corrosion resistance and high temperature strength.
The present invention provides a titanium aluminide material alloyed with certain selected alloying elements in certain selected proportions that Applicants have discovered yield an unexpected improvement in alloy creep resistance while maintaining other alloy properties of interest.